Investment mold and core assembly



y 1956 D. K. HANINK ET AL INVESTMENT MOLD AND coRE ASSEMBLY Filed Feb.24, 1953 Attorneys United States Patent INVES'IWNT MOLD AND COREASSEMBLY Dean K. Hanink, Birmingham, and Robert E. Clague,

Berkley, Mich assignors to General Motors Corporalion, Detroit, Mich, a.corporation of Delaware Application February 24, 1953, Serial N 0.338,334

13 Claims. (Cl. 22196) This invention relates to investment molding andparticularly to a process for forming an investment mold and coreassembly which is especially suitable for precision casting operations.

Inasmuch as it is exceedingly difficult to maintain close dimensionaltolerances in precision casting operations using mold investments, it isdesirable that any internal refractory core be located and supported insuch a manner so that suitable space is provided at the core printlocations to allow for difierential thermal expansion and contraction ofthe mold and core. This differential movement between the core andinvestment material can be caused by non-uniform heating of the entiremold assembly from the exterior to the interior, localized shock heatinggenerated by the molten metal during the pouring operation, or simply bythe use of dissimilar materials in the investment and core. In theabsence of such a provision for differential expansion of the mold andcore, stresses will be set up within thecore which result in upsettingand subsequent failure of the core. Heretofore this problem has not beensatisfactorily solved, and methods previously employed for theproduction of precision castings requiring the use of cores, such ashollow turbine buckets, have involved excessively high costs.

A principal object of the present invention, therefore, is to provide aninvestment mold and core assembly for precision casting operations whichprovides for differential expansion and contraction of the assemblyparts. Precision castings which necessitate the use of cores thus may beaccurately produced in accordance with the invention because thisassembly eliminates core upsetting and breakage due to thermal stresses.A further object of this invention is to provide a process for forming amold and core assembly which eliminates the v disadvantages of high costinherent in precision casting methods heretofore used by employingcommon foundry materials which are both inexpensive and adaptable toquantity production with the use of standard foundry equipment.

These and other objects are attained in accordance with the presentinvention by an improved investment mold and core assembly, and aprocess for forming the same, for the precision casting of metal parts,particularly hollow parts requiring the use of cores. This isaccomplished by investing a sand core and a destructible pattern Withina refractory composition in a manner so as to provide for differentialexpansion and contraction between the core and the investment material.

In accordance with the invention, an improved investment mold and coreassembly possessing the above desirable characteristics may be formedfrom common foundry materials and equipment by a process in which aninternal core is first embedded within a destructible pattern, the coreprint projections subsequently coated with wax or other equivalentdestructible material to provide for the aforementioned differentialthermal expansion and contraction, and the formed core and patternassembly 2,756,475 Patented July 31, 1956 ice 2 thereafter invested in aconventional manner in a refractory mold.

Other objects and advantages of this invention will more fully appearfrom the following detailed description of the invention, referencebeing made to'the accompanying drawing which shows a somewhat schematicsectional view of a sand core and a destructible pattern invested in arefractory mold in accordance with the invention.

Referring more particularly to the drawing, an internal sand core 10,which preferably has been previously baked, is positioned in aninjection mold. Subsequently, wax or other destructible pattern materialis injected into the mold under pressure in a conventionalmanner. Uponsolidification of the wax, the core, together with the surrounding waxpattern 12, is removed from the mold.

Although the pattern 12 is generally described herein as being a waxpattern, it also may be formed of any other suitable low fusingsubstance, such as a thermoplastic resinous material or any othervaporizable, fusible, combustible or otherwise destructible material. Infact, plastic patterns are frequently employed to provide optimumresults. Among the plastic patterns which have been found to besatisfactory are those formed of polystyrene, although otherthermoplastic pattern materials, such as resinous polymerizedderivatives of acrylic acid and resinous polymerized derivatives ofmethacrylic acid, may be used.

After removal of the formed core and pattern subassembly from theinjection mold, the pattern is preferably cleaned with an alcoholsolution and air dried. Wax 14 is then applied over the entire surfacesof the core print projections or locators 16 and 18 to a thickness whichwill subsequently permit the satisfactory dilferential expansion andcontraction of the parts of the formed investment mold assembly. We havefound that the thickness of this wax coating 14 preferably should not begreater than approximately one-half the dimensional tolerance of thecasting wall thickness. For example, in the case of a turbine bucket, ifthe tolerance of the adjacent opening formed by the core is 0.012 incheson the diameter, the wax layer should not'be thicker fl'ian 0.003inches. While the thickness of the wax coating is somewhat exaggeratedin the drawingfor pur- Following the application of wax to the coreprint' projections 16 and 18, a suitable gating portion 20 having ariser and pouring basin part 22 at its outer end is attached to thepattern in which the core. 10 is embedded.

The portions 20 and 22 may be formed of wax, plastic or otherappropriate destructible material similar-to that used to form thepattern. Of course, if convenient for the particular application, all orpart of the gates, risers and/or headers may be formed integral with thepattern in the injection molding machine. Likewise, although side gatingis shown as being used in the drawing to cast a hollow turbine bucket,end gating may be alternatively employed if the bucket design sowarrants.

The surfaces of the pattern are next coated with a ceramic wash orcoating material 24 which is to provide the smooth casting surface ofthe refractory mold to be formed. This coating material comprises anaqueous,

Coating of the pattern with the ceramic wash is pref-.

erably accomplished by dipping the pattern in the coating solution.Although in some instances the ceramic coating may also be applied byspraying or painting it on the pattern or in any other suitable manner,dipping is preferred because it assures moreuniform coating of all thepattern surfaces and is the simplest method of application. J i i Thedip coat slurry is preferably kept in constant motion by stirring meansexcept during the actual-dipping operation. However, the mixing actionshould not be such as to unnecessarily introduce air into the 'slurry,and care should be exercised in immersing the pattern in the slurry toprevent air entrapment on the pattern. Normally the dip coat solution isretained at room temperature during the dipping operation becauseexcessive heat can result in distortion of the plastic or'wax' pattern.The excess coating material is permitted to drain olf prior tosubsequent treatment and investment.

After the pattern has been completely coated with the dip coat slurry,it may be sanded or stuccoed to provide a rough surface on the coating,thus insuring greater adhesion between the principal refractory portionof the mold and the dip coat on the pattern. This sanding may beaccomplished by merely screening or otherwise applying silica sand orother suitable refractory materials in known manner to the outer coatedsurface of the destructible pattern. When all the molding surfaces ofthe pattern have been effectively covered with sand, the pattern andembedded core should be air dried.

Following the formation and treatment of the core and pattern assembly,suitable investment material 26, usually containing a coarser refractorymaterial, is formed about the pattern and sprue, the latter beingpermitted to extend through the wall of the resultant refractory mold soas to permit the escape of the destructible pattern and to form aningate for the fluid casting meta-h This refractory mold may be formedabout the pattern in any suitable manner, but the following preferredprocedure provides excellent results. A base plate 28 is preferablyfirst sprayed or otherwise coated with molten wax so as to form a thinfilm of wax over its upper surface. Before the wax has completelysolidified, the pattern and core assembly to be invested is positionedon the plate 28 with the gate and pouring basin portions 20 and 22thereof extending downwardly and seated firmly in the wax'film. A sleeveor flask 30, preferably of steel, is then placed around the pattern andpressed lightly into the wax layer. In order to completely seal theflask '30 to the plate 28, it is preferable to again spray molten waxaround the outer surfaces of these parts at their junction. This waxshould then be permitted to thoroughlysolidify. Y

The refractory material is mixed with a predetermined quantity of liquidbinder, and the resultant slurry ;is poured into the sleeve or flask 30,which is preferably vibrated during this pouring operation, the mold 26then being permitted to set. The mold body 26 maybe formed 'of aconventional silica investment having an ethyl silicate Y binder or maybe formed of any other suitable investment material. An example of aninvestment dry mix or grog which may be used is one comprisingmajorproportions ofafinely ground, dead burned fire clay and silica flour andminor proportions of magnesium 'oxide and hora-x glass. The binder forthis grog may include anaqueous solution of condensed ethyl silicate,ethyl alcohol and bydrochloric acid. 7

When the mold body 26 has solidified or set to a sulfic'ient extent, thebase plate 28 is removed "from beneath the mold and heat is applied tomelt the destructib'le pattern 12 and wax coating 14 on the core printprojections. It'is necessary to apply sufficient heat to raise the moldtemperature above the fusing points of these materials, thus permittingthe molten pattern and coating Ito-escape through the gate .and spruepassage in the mold formed by the .removed gating and riser portions-l0.22, irespectively. When a wax pattern and coating are employed, atemperature of approximately 400 F. has been found to be satisfactory.In this manner the dip coat which had covered the pattern tightlyadheres to the remainder of the mold and provides the casting cavitywith a smooth coating. It is also possible to vaporize the pattern andcoating, if vaporizable materials are used, by heating the mold rapidlyto a high temperature.

After removal of the pattern from the mold in the foregoing manner, thecore 10 remains within the mold with its body portion extending throughthe formed mold cavity. The core print projections 16 and 18 are stillpositioned in the core prints 32 and 34 which were previously formed inthe investment slurry by the wax-coated core print projections. The moldassembly is then burned out to remove substantially all the volatilematter. This assembly is preferably preheated to thedesired-temperature, and the molten casting metal is poured or otherwiseintroduced into the mold cavity formed around the core by the fusiblepattern. In the majority of instances it is necessary to pour thecasting metal while the mold is still hot. After the molten metal hasbeen poured and the casting has solidified, the refractory mold body 26and the adhering coating 24 may be broken to permit the removal of thecasting.

After the wax layer 14 has been melted off or otherwise removed frombetween the core prints 32 and 34 and the core locators 16 and 18, thecore is free to expand independently of the mold proper to a limitedextent. Hence, during the pouring operation this core is not subjectedto the thermal stresses which would otherwise result in upsetting andfailure of the core. The degree of accuracy of castings obtained by thisarrangement, of course, is dependent upon the space provided at the coreprints for dilferential movement between the core and mold. It will benoted, therefore, that the thickness of the layer of wax applied to thecore print locations determines the accuracy of the final location.

Although the spacing layers 14 are herein generally referred to asformed of wax, it will be appreciated that other destructible materials,such as those referred to as appropriate pattern materials, may also beemployed under many conditions to coat the core print projections.

Hence, any suitable substance, suchas for example, resinin the resultingarrangement is that the core is not rigidly retained in its initialposition by the investment or any other mold component which is fixed orfastened tothe investment. In the selection of a wax, resin or any otherdestructible material to be used as the spacing coating,

it is important that such materials will melt, volatilize or burnattemperatures not in excess of approximately 1800 F. and preferably atconsiderably lower temperatures.

The core and mold assembly formed in the abovedescribed manner maybeused without limitation as to the typeof metal to be-cast. Hence, thisassembly maybe successfullyemployedwith, for example, both low-meltingferrous metals and high-melting cobalt-based-super alloys, thelattercurrently frequently being used to form turbine buckets. Theprocess for forming the mold assembly does not adversely affect coreproperties, the cores retaining their desirable permeability andcollapsing characteristicswhen used in this manner. 7

While .the present invention has been described by means of certainspecific examples, it will be understoodthat the scope of the inventionis not to be limited thereby except as defined in'the following :claims.i

We claimf .1. A .method of forming an investment .mold and core assemblywhich comprises embeddin a core :within a fusiblepattern,coatingza'core-print projectionon said core with a layer -.of fusiblematerial .tor ov e for expansion and contraction of said core,thereafter investing said core and pattern in a refiactory mold, andsubsequently removing the pattern and material from the refractory mold.

2. A method of forming an investment mold and core assembly whichprovides for differential expansion and contraction between the core andinvestment material, said method comprising embedding an internal corewithin a fusible pattern, coating core print projections extending fromsaid core through said pattern with a layer of fusible material,thereafter introducing a refractory molding composition around said coreand pattern, permitting said molding composition to solidify, andfinally removing the fusible pattern and layer of fusible material fromthe formed investment mold and core assembly.

3. A method of forming an investment mold and core assembly whichcomprises molding a fusible material around a sand core into the form ofa pattern, applying a layer of fusible material to the surfaces of coreprint projections on said core, subsequently applying a thin refractorycoating to the surfaces of said pattern, thereafter investing the coreand coated pattern assembly in a refractory investment material, andfinally removing the pattern and fusible material from within saidinvestment material to provide a casting cavity within the formedinvestment mold and around said core.

4. A method of forming a refractory mold and core assembly whichprovides for difierential thermal expansion of the core and mold, saidmethod comprising inserting a refractory core in an injection mold,injecting a combustible material selected from the class consisting ofwax and plastic at least partially around said core and molding saidmaterial into the shape of a pattern for the casting to be formed,coating the surfaces of projections on said core with a layer ofcombustible material, said layer having a thickness not greater thanone-half the dimensional tolerance of the casting wall thickness,thereafter applying a thin coating of a fine refractory mixture to thesurfaces of said pattern, investing said coated pattern and core in arefractory mold, and thereafter eliminatmg the pattern and layer ofcombustible material around said core print projections from said mold,whereby said refractory coating adheres to the refractory mold and thecore remains positioned within said cavity.

5. The process of forming a refractory mold and core assembly in which acore is positioned within a mold cavity and has core print projectionsspatially separated from core prints formed in said mold, said processcomprising forming a fusible pattern around a refractory core, coatmgcore print projections formed on said core with a thin layer of fusiblematerial, thereafter investing said coated pattern and core in a body ofinvestment material, setting the body of investment material, andthereafter melting and removing the pattern and layer of fusiblematerial, whereby the core print projections become spatially separatedfrom the core prints formed in said refractory mold to permit thermalexpansion and contraction of said core relative to said mold.

6. A method of forming an investment mold and core assembly whichcomprises positioning a sand core in a Wax injection mold, injecting waxinto the mold to form a pattern around said core, thereafter removingthe core and surrounding Wax pattern from said mold, applying a layer ofwax to the surfaces of core print projections formed on the core,assembling and attaching destructible gates and risers to the patternand sand core assembly, subsequently coating the pattern with a ceramicwash, thereafter stuccoing said wash with silica sand and drying thecoated and stuccoed pattern and core assembly, investing said assemblywithin a refractory molding composition, thereafter elimina g the WaxPattern and Wax layer by heating the assembly to a temperature above themelting point of the wax, and subsequently preheating the mold to thedesired temp Prior to casting- 7. A process for forming an investmentmold and core assembly which provides for differential expansion andcontraction of the core and investment material, said process comprisingpositioning a sand core in a wax injection mold, injecting a fusiblematerial into the mold into the form of a pattern around said core,thereafter removing the core and surrounding wax pattern from said mold,applying a layer of a fusible material over the entire surfaces of coreprint projections formed on said core, said layer having a thickness notgreater than approximately one-half the dimensional tolerance of thecasting wall thickness, assembling and attaching fusible gates andrisers to the pattern and sand core assembly, subsequently coating thesurfaces of the pattern with a ceramic wash, thereafter applying a smallamount of silica sand to the surfaces of said wash, thereafter dryingthe coated and sanded pattern and core assembly, introducing arefractory molding composition around said assembly, permitting saidmolding composition to solidify, removing the pattern and layer offusible material by heating the formed mold and core assembly to atemperature of at least approximately 400 F., and subsequentlypreheating the mold and core assembly to the desired temperature priorto castmg.

8. A refractory mold assembly comprising a body of refractory materialprovided with a casting cavity, and a core positioned in said cavity andextending into a core print in the casting cavity walls of said body ofrefractory material, said core being generally separated from the wallsof said core print an average distance equal to approximately one-halfthe dimensional tolerance of the casting wall thickness to permitdifferential expansion of said core and body of refractory material.

9. A refractory mold and core assembly comprising a body of refractorymaterial provided with a casting cavity therein, and a core positionedin said cavity and having core locators projecting into core prints inthe casting cavity Walls of said body of refractory material, said corelocators being generally separated from the walls of said recesses adistance sufiicient to permit differential thermal expansion andcontraction between said core and body of refractory material but notappreciably greater than approximately one-half the dimensionaltolerance of the casting wall thickness.

10. An investment mold and core assembly for precision castingoperations comprising a body of refractory material provided withcasting cavity walls having a smooth refractory coating thereon, saidwalls having recesses formed therein, a refractory core positionedwithin said mold cavity and provided with core locators engaging saidrecesses, said core locators being generally separated from the Walls ofsaid recesses by a thin layer of combustible material which is removableto allow subsequent differential thermal expansion and contraction ofsaid core and mold.

11. An investment mold and core assembly for precision casting of hollowturbine buckets, said assembly comprising a mold formed of a comminutedrefractory material provided with a casting cavity therein, a baked sandcore positioned within said cavity and having locating portionsextending into core prints formed in said mold, a destructible patternat least partially surrounding said core, and a thin layer of a fusiblematerial interjacent the mating surfaces of the core prints and thelocating portions of said core, the surfaces of said pattern beingprovided with a smooth refractory coating.

12. A refractory mold assembly comprising a body of refractory materialprovided with a casting cavity, a core positioned in said cavity andextending into a core print in the casting cavity walls of said body ofrefractory material, and a layer of fusible material interjacent themating surface of the core print and the core positioned therein.

13. An investment mold and core assembly for precision castingoperations comprising a body of refractory material rovided with .acasting cavity therein, a bak d san core positioned within sai a i y andh ng loca ng portions extending into core prints formed in said mold, afusible pattern selected from the class consisting of wax and plastic atleast partially surrounding said core, and a thin layer of wax providedbetween the mating surfaces of the core prints and the locating portionsof said core.

References Cited in the file of this patent UNITED STATES PATENTS eanm-a- ,--V-.- ay 1948

1. A METHOD OF FORMING AN INVESTMENT MOLD AND CORE ASSEMBLY WHICHCOMPRISES EMBEDDING A CORE WITHIN A FUSIBLE PATTERN, COATING A COREPRINT PROJECTION ON SAID CORE WITH A LAYER OF FUSIBLE MATERIAL TOPROVIDE FOR EXPANSION AND CONTRACTION OF SAID CORE, THEREAFTER INVESTINGSAID CORE AND PATTERN IN A REFRACTORY MOLD, AND SUBSEQUENTLY REMOVINGTHE PATTERN AND MATERIAL FROM THE REFRACTORY MOLD.